In transmission X-ray image acquisition, an object to be examined, e.g., a patient, is arranged between an X-ray generating device, for example an X-ray tube, and an X-ray detector.
X-ray radiation emanating from the X-ray generating device penetrates the object to be examined and subsequently arrives at the X-ray detector for acquiring image information, which may later then be reconstructed into an X-ray image for presentation. The inner structure of the object, e.g., the tissue structure, provides spatial attenuation of the X-radiation penetrating the object. Accordingly, the X-ray detector is registering the spatially attenuated X-ray radiation.
Certain objects may attenuate X-ray radiation only to a smaller extent or may rather uniformly attenuate the X-ray beam resulting in an acquired X-ray image having low contrast.
However, even objects imposing only a small amount of attenuation of an X-ray beam penetrating the object, a phase of a wave front of X-ray radiation may be influenced to a rather large extent by the same object.
Accordingly, phase-contrast imaging may be employed for obtaining image information of an object with enhanced contrast.
In phase-contrast imaging, an X-ray source together with a so-called source grating element arranged adjacent to the X-ray source generates at least partly spatially coherent X-ray radiation. Coherent X-ray radiation penetrating the object may allow a subsequent retrieval of phase information.
Since a phase of a wave may not be measured directly, a further grating element, a so-called phase grating, is employed, arranged between the object to be examined and the X-ray detector. The phase grating allows for a conversion of a phase-shift to an intensity modulation by interference of a plurality of waves, which intensity modulation may then be detectable by an X-ray detector.
However, an interference pattern generated by employing a phase grating only may be too small for a current X-ray detector to be precisely detectable, due to a lack of spatial resolution of the X-ray detector. Here, an additional grating element, a so-called analyzer grating, may be employed, which is arranged between the phase grating element and the X-ray detector in the vicinity of the X-ray detector. The analyzer grating provides an interference pattern, which is large enough to be detectable by a current X-ray detector.
To obtain appropriate phase-contrast image information, phase stepping is conducted for obtaining a plurality of phase-contrast projections. In phase stepping, one of the source grating element, the phase grating element and the analyzer grating element is displaced laterally with respect to the other gratings and the X-ray detector element by a fraction of the respective grating pitch, e.g., a fourth, sixth, eighth of the grating pitch of, e.g., the phase grating. Image acquisition and lateral displacement is repeated, e.g., four, six, or eight times, for acquiring a plurality of phase contrast projections, constituting together a phase stepping interval.
In differential phase-contrast imaging, the first derivative of a phase front perpendicular to the grating direction of a grating element, i.e., the extension of the barrier regions and the trench regions of the grating structure, is detected, thus measured, by the X-ray detector. Due to the grating structure, the acquired image information may be considered to be highly asymmetric with edges being enhanced in particular in one direction, i.e., the direction perpendicular to the grating direction. The direction parallel to the grating direction may not be enhanced.
For reconstructing image information, an integration procedure along the lines of differentiation, i.e., perpendicular to the grating structure, may result in image information having streak-like artefacts or streaks due to noise or other errors, which are arranged locally in the differential image information and which may propagate along the line of reminder of the image data.
Thus, it may be beneficial to provide means for reducing or even removal of said streak-like artefacts.
Phase-contrast imaging is described in both Weitkamp T., Diaz A., David C. et al.: “X-ray phase imaging with a grating interferometer”; Optics Express 6296, 8. August 2005/vol. 13, no. 16 as well as Bartl P., Durst J., Haas W. et al. “Simulation of X-ray phase-contrast computed tomography of a medical phantom comprising particle and wave contributions”, Proc of SPIE vol. 7622 76220Q-1.
Determining regularization parameter is described in Engl H. W. and Greyer W.: “Using the L-curve for determining optimal regularization parameters”; Numer. Math. 69: 25-31 (1994).